The present invention relates to a nuclear fuel assembly which is loaded with nuclear fuel elements in which a burnable poison is incorporated and more particularly it concerns a nuclear fuel assembly having little possibility of nuclear fuel elements within which burnable poison is added failing.
A nuclear fuel assembly used in a boiling water reactor comprises nuclear fuel elements, an upper tie-plate, a lower tie-plate, a channel box and a spacer. The nuclear fuel element is formed by loading plural UO.sub.2 pellets within a fuel cladding, providing an end plug at both ends of the fuel cladding and sealing them by welding. The UO.sub.2 pellets are formed by molding powders of uranium dioxide and sintering it. Zircaloy-2 is used for the fuel cladding. Within the nuclear fuel assembly, roughly classified, two kinds of nuclear fuel elements are loaded. One of them is a nuclear fuel element which comprises UO.sub.2 pellets to which gadolinium oxide (Gd.sub.2 O.sub.3), a burnable poison, is added (which is referred to as gadolinium-nuclear fuel element hereinafter). Another one is a nuclear fuel element which comprises UO.sub.2 pellets to which no gadolinium oxide is added (which is referred to as non-gadolinium-nuclear fuel element hereinafter). Addition of gadolinium oxide to UO.sub.2 pellets is for controlling excess reactivity of the nuclear fuel assembly at the beginning of burning of the nuclear fuel assembly and smoothing the power of nuclear fuel assembly in the horizontal direction. About 2-5 gadolinium-nuclear fuel elements are loaded within the nuclear fuel assembly.
As the results of various researches on characteristics of a nuclear fuel assembly constructed as mentioned above, it has been found that the gadolinium-nuclear fuel element more easily fails than the nongadolinium-nuclear fuel element. When nuclear fuel elements fail, fissionable material present therein, and fission products accumulated therein by nuclear fission, leak out of the nuclear fuel element. This is apt to cause environmental contamination and markedly reduces the safety of nuclear reactor. The nuclear fuel assembly containing failed nuclear fuel element must be substituted with fresh nuclear fuel assembly. This causes prolongation of shut-down periods of nuclear reactors and reduction in operation rates of the nuclear reactors.